Re: Update _FloatN header support for C++ in GCC 13

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Sep 28, 2022 at 12:23:31AM +0000, Joseph Myers wrote:
> GCC 13 adds support for _FloatN and _FloatNx types in C++, so breaking
> the installed glibc headers that assume such support is not present.
> GCC mostly works around this with fixincludes, but that doesn't help
> for building glibc and its tests (glibc doesn't itself contain C++
> code, but there's C++ code built for tests).  Update glibc's
> bits/floatn-common.h and bits/floatn.h headers to handle the GCC 13
> support directly.
> 
> In general the changes match those made by fixincludes, though I think
> the ones in sysdeps/powerpc/bits/floatn.h, where the header tests
> __LDBL_MANT_DIG__ == 113 or uses #elif, wouldn't match the existing
> fixincludes patterns.
> 
> Some places involving special C++ handling in relation to _FloatN
> support are not changed.  There's no need to change the
> __HAVE_FLOATN_NOT_TYPEDEF definition (also in a form that wouldn't be
> matched by the fixincludes fixes) because it's only used in relation
> to macro definitions using features not supported for C++
> (__builtin_types_compatible_p and _Generic).  And there's no need to
> change the inline function overloads for issignaling, iszero and
> iscanonical in C++ because cases where types have the same format but
> are no longer compatible types are handled automatically by the C++
> overload resolution rules.
> 
> This patch also does not change the overload handling for iseqsig, and
> there I think changes *are* needed, beyond those in this patch or made
> by fixincludes.  The way that overload is defined, via a template
> parameter to a structure type, requires overloads whenever the types
> are incompatible, even if they have the same format.  So I think we
> need to add overloads with GCC 13 for every supported _FloatN and
> _FloatNx type, rather than just having one for _Float128 when it has a
> different ABI to long double as at present (but for older GCC, such
> overloads must not be defined for types that end up defined as
> typedefs for another type).
> 
> Tested with build-many-glibcs.py: compilers build for
> aarch64-linux-gnu ia64-linux-gnu mips64-linux-gnu powerpc-linux-gnu
> powerpc64le-linux-gnu x86_64-linux-gnu; glibcs build for
> aarch64-linux-gnu ia64-linux-gnu i686-linux-gnu mips-linux-gnu
> mips64-linux-gnu-n32 powerpc-linux-gnu powerpc64le-linux-gnu
> x86_64-linux-gnu.
> 
> ---
> 
> I am seeking review of this patch, not applying it as a math
> maintainer.

Went through all these and compared them to my include-fixed
of whole libc tree and the only changes are the _Float128x errors
which is desirable.
So, LGTM.

Reviewed-by: Jakub Jelinek <jakub@redhat.com>

	Jakub